A rear seat of an automobile is known in which a frame of a seat back includes a panel frame fixedly installed on a partition panel of a vehicle body (Japanese Patent No. 4146135). The panel frame is integrally fixed in such a way that two left and right portions on the lower end side and one central portion on the upper end side are respectively bolted on the partition panel. A bead of a drawn shape for enhancing rigidity against bending and torsion of a panel surface is formed over a wide extension range of the panel surface of the panel frame.
However, in the above related art, when any load acting portion such as an operation lever or a headrest support portion is set in either a left shoulder portion or a right shoulder portion on the upper end side of the panel frame, a load inputted from the load acting portion causes an overload at a position on a line connecting upper and lower fixed points of the panel frame that supports the same load as a cantilever load. Thus, there is a possibility that the panel frame is deformed into a bent shape.